This invention is related to copending application Ser. No. 791,894 by Michael E. Long which is assigned to the assignee of the present invention.
This invention relates generally to television AFC systems. The television broadcast signal comprises a carrier upon which luminance, chrominance and sound information is modulated within a limited bandwidth. The sound information is present as a frequency-modulated subcarrier displaced from the picture carrier. In the NTSC signal transmission used in the U.S. of America, the sound and picture carriers are frequency-spaced 4.5MHz while the channel bandwidth is 6MHz. In order to fit the frequency-spaced sound and picture carriers and other information within the prescribed bandwidth, vestigial transmission in which part of one sideband (in this case the lower sideband) is substantially attenuated with respect to the other sideband (upper sideband) is used. The range of broadcast frequencies assigned to television transmission is not continuous but rather forms an interrupted group of bands. However, despite this discontinuity, all assigned channels may have at least one adjacent channel, and often two adjacent channels may "flank" each channel. This means that the majority of channels may have adjacent sound carrier signals 6MHz above and below their desired or associated channel sound carrier and adjacent picture carriers 6MHz above and below their associated channel picture carrier. This condition often exists on cable distribution systems.
The vast majority, if not all, of currently manufactured television receivers include a tuning system which selects a desired channel by frequency converting the received broadcast signal, using the well-known heterodyning process, to a common intermeidate frequency (IF) signal having frequency-spaced picture and sound carriers. Unfortunately, the heterodyning process also frequency converts the adjacent channel carriers to "intermediate frequency" signals. As a result, most receivers use frequency selective intermediate frequency filters which pass the desired intermediate frequency signal but include trap networks which exclude or attenuate undesired adjacent channel information.
For example, in the system of assigned frequencies within the U.S. a standard IF frequency of 45.75MHz for the picture carrier has been generally established. Correspondingly, the associated sound carrier is 4.5MHz lower in frequency at 41.25MHz. In the portions of the television band in which adjacent channels are present, the IF sound carrier of the lower adjacent channel is 47.25MHz ( only 1.5MHz away from the desired channel picture carrier) while the picture carrier of the upper adjacent channel is 39.75MHz (only 1.5MHz away from the desired channel sound carrier). This situation has lead practitioners in the television art to flank the IF passband with trap networks to attenuate the adjacent signal carriers. Unfortunately, these traps also upset associated channel carrier relationships during mistuning.
These and other stringent tuning requirements have lead to the use of automatic frequency control systems (AFC) to maintain correct frequency conversion within the tuning system. Such AFC systems are well-known in the art and are of almost endless variety but all may be characterized by the performance of two essential functions. The first function is generally called "pull-in" or "frequency acquisition" in which an existing frequency deviation of the picture IF carrier from the desired 45.75MHz is corrected by the closed-loop response of the AFC system. The second function is generally called "hole-in" which involves the maintenance of correct frequency conversion once frequency acquisition has been accomplished. A basic limitation in the ability of most AFC systems to acquire correct tuning in the face of a substantial frequency deviation arises from the presence of the adjacent channel picture and sound carrier traps mentioned above. For example, should the oscillator frequency be displaced such that the frequency conversion results in "placing" the video carrier within the adjacent channel sound trap, virtually no error signal, or control effect, is produced within the AFC system due to the picture carrier. However, the sound carrier under such conditions is "exalted" by the IF filter response and produces substantial energy within the AFC system often resulting in an erroneous control voltage.
In most AFC systems, dominance by the sound carrier rather than the picture carrier, causes the system to lose its pull-in capability and "lock-out" of the system occurs. The relative signal strengths of picture and sound carriers is determined by the IF filter response, and under proper transmission conditions and near correct tuning the effect of the picture carrier will dominate the AFC system. However, transmission problems such as multi-path interference or "tilt" within the antenna and distribution system can disturb this amplitude relationship resulting in the production of an overriding sound carrier control effect which again can cause a lock-out condition.
The problem of AFC lock-out caused by the intrusion of the picture carrier into the adjacent channel sound trap has been minimized by development of AFC systems in which the sound carrier produces a control effect of the proper polarity to aid or complement that produced by the picture carrier and actually control the AFC system when the picture carrier is substantially attenuated. One such system shown in U.S. Pat. No. 3,459,887 uses an automatic frequency control system in which the balance of a diode-pair AFC detector is offset, or biased, to produce the desired complementary control effect by the sound carrier. The described system achieves substantial improvement in AFC pull-in when the picture carrier is attenuated by the adjacent channel sound trap. A somewhat similar system is shown in U.S. Pat. No. 3,968,325 in which a product detector, or multiplier, simultaneously driven by a pair of IF signals emmanating from the intermediate frequency filter performs the AFC detection function. A frequency-dependent phase shift between the two IF signals is introduced such that the frequency deviation of the intermediate frequency signal is converted to a phase deviation to which the product detector responds. The AFC response provides a reduction of the erroneous AFC voltage produced by detected noise in the region of the received channel sound carrier and a complementary sound carrier control effect similar to that of the U.S. Pat. No. 3,459,887. The creation of a complementary sound carrier control effect in both systems provides improved pull-in or acquisition when the frequency deviation is such that the picture and sound carriers are above the correct frequencies. However, such systems do not produce complementary control effects when the frequency deviation is low.
The above-noted related application sets forth a novel AFC system in which an improved complementary control effect even at small frequency deviations as well as the avoidance of a frequency off-set is provided. The described system in its preferred form utilizes an additional resonant network in the frequency-dependent phase shift network to provide an AFC characteristic having zero crossing at the intermediate frequency sound carrier.
In addition, a number of phase shift type frequency discriminators have been developed in the art. Representative of which are U.S. Pat. Nos. 3,582,540 and 3,714,594 both assigned to the assignee of the present invention.